In a waterflood project for recovery of additional oil after completion of primary production, water is injected through at least one injection well extending into an oil-bearing formation to displace formation fluids, including oil, and to produce additional oil from at least one producing well extending into the flooded formation. The displaced fluids recovered through the production well include a mixture of oil and water. This mixture is typically separated into an oil stream and a water stream by conventional means such as settling, coalescing, or the like. Most commonly, the separation is carried out in a heater-treater of the type widely used in oil production operations.
In a waterflood operation, the separated oil is recovered, and the separated water, which invariably contains a small but significant amount of oil even after the separation step, is reinjected into the formation as additional flooding water.
The presence of this small amount of oil in the separated water stream (oily water) is detrimental in several respects. The oil in the oily water, if not separated, is lost from the recovered oil total. Even though the concentration of oil in the oily water is low, the volume of water is high, such that the amount of lost oil recovery over a period of time is significant. Additionally, the presence of oil in the injection water leads to injection inefficiencies. Even small amounts of oil in injection water can substantially reduce the water injectivity at a given level of pumping capacity, largely due to permeability effects on the formation adjacent the injection well.
It is technically feasible to reduce the amount of oil in the oily water stream to acceptable levels using conventional techniques, but the equipment needed to accomplish this by, for example, use of heater treaters and separation tanks, is too large and heavy to be acceptable for offshore operations where platform weight and space are limiting factors.
One recent proposal for overcoming the problem relies on use of hydrocyclones on the production platform to provide a high degree of separation of the oil and water. This approach has met with considerable success, since the weight and space requirements for the hydrocyclones are much less than for conventional separators. However, the system has limitations in that some oily water streams are difficult to separate in hydrocyclones because of surface tension or density considerations. For example, the density of some produced oils is close enough to that of water to limit the efficiency of the hydrocyclone.
Thus, there is a continuing need for more efficient methods of recovering oil from oily water (defined herein as water or brine containing less than 2 percent oil by volume). The present invention provides such a method.